Much research into either biochemical abnormalities associated with depression and related psychiatric disorders or the mechanism(s) of action of psychotropic drugs has focused on brain monoamines, in particular norepinephrine and serotonin (5HT). The tricyclic antidepressants and some of the newer second generation drugs are potent inhibitors of the neuronal uptake of norepinephrine (NE) and/or serotonin. These effects are important since neuronal uptake is the primary inactivating mechanism for released monoamines and alterations in neuronal uptake may be related to the clinical action of these drugs. A major advance in recent years has been the identification of high affinity binding sites for tricyclic antidepressants, in particular, sites in the brain to which labelled imipramine binds. These binding sites have been found to be associated with uptake sites for 5HT. The development of this tool has facilitated greatly our ability to study the regulation of the 5HT uptake site and whether it shows alterations in neuropsychiatric disorders. Post-mortem samples from suicides have shown a decreased density of sites in brain compared with control samples. Also, imipramine binding sites are present on human platelets and several studies have shown a decrease in the density of these sites in depressed patients. The NE uptake site in brain has been studied far less than the uptake site for 5HT. Most probably, this is due to the absence of a suitable radioligand for this site. So far, studies have been done with labelled desipramine and more recently with labelled mazindol. Labelled desipramine is not an ideal ligand for labelling uptake sites associated with NE as it gives a high amount of non-specific binding and binds to low affinity sites in addition to high affinity sites. These low affinity sites are unrelated to NE uptake sites. Labelled mazindol is also not suitable for labelling uptake sites for NE since in most brain areas only a small proportion of its binding is to these sites. Both ligands are unsuitable for studying the NE uptake sites in vivo as the binding in vivo decreases at physiological temperatures. We propose to develop radioligands for the measurement of sites associated with the uptake of NE in the brain. Compounds selected for study have been reported previously to have high affinity and selectivity for NE uptake sites. Potential ligands will be radiolabelled with tritium to high specific activity and their binding properties will be investigated both in vitro and in vivo. In vitro binding will be assessed using both homogenate preparations and quantitative autoradiography of slide-mounted thin brain sections. If successfully developed, such a ligand would allow many types of studies to be performed on this uptake site, analogous to those reported already on the uptake site for 5HT.